At some point in my workflow, I need to make sure the issue has been validated/attested/signed off on. Can I do this in VisionProject?

Yes. I'll outline a simple setup below.

You will basically have to create a narrow point in your workflow where all issues have to go to one status before it can go through to others. Additionally, you want it so that only specific users can attest the issue and change the status to something else.

Create a new User group 'Attester'. Copy this group from a suitable existing group, and adjust role, settings and permissions. (General -Settings - User groups)

Create a new status for the restricted point in the workflow. Let's call this status 'For attesting'. (General Settings - Issue statuses - "Add new status")

Add the status to the project in question. (Project Settings - General - Statuses).

Create a workflow for all statuses in the project outlining all valid paths. (Project Settings -Workflow, the section 'Valid status changes'). It will make it easier if you sketch out the status relationships first, before configuring. Make sure that the workflow passes through the 'For attesting'.

Make sure that only the 'Attester' user group can change the status, by setting the group access on the status to 'Unable to change the status' for all user groups in the project, except 'Attester', where you set 'No limitations'. (Project Settings - General - Statuses, expand the "edit access" for the status).

Now you should have a workflow passing through the status 'For attesting', where only users of the user group 'Attester' can change the status to something else. It should be apparent in the issue history who did the attest and when (by noting when the status changed from 'For attesting')